Cloud Seeding 101

There are three cloud seeding methods: static, dynamic and hygroscopic.

Static cloud seeding involves spreading a chemical like silver iodide into clouds. The silver iodide provides a crystal around which moisture can condense. The moisture is already present in the clouds, but silver iodide essentially makes rain clouds more effective at dispensing their water.

Dynamic cloud seeding aims to boost vertical air currents, which encourages more water to pass through the clouds, translating into more rain [source: Cotton]. Up to 100 times more ice crystals are used in dynamic cloud seeding than in the static method. The process is considered more complex than static clouding seeding because it depends on a sequence of events working properly. Dr. William R. Cotton, a professor of atmospheric science at Colorado State University, and other researchers break down dynamic cloud seeding into 11 separate stages. An unexpected outcome in one stage could ruin the entire process, making the technique less dependable than static cloud seeding.

Hygroscopic cloud seeding disperses salts through flares or explosives in the lower portions of clouds. The salts grow in size as water joins with them. In his report on cloud seeding, Cotton says that hygroscopic cloud seeding holds much promise, but requires further research.

The Beijing Weather Modification Office spent a lot of time researching how to prevent rain in the city during the Aug. 8 opening ceremony of the 2008 Summer Olympics. The government even guaranteed clear skies for the event — a promise it managed to deliver on. The feat only took the launch of 1,104 rain dispersal rockets from 21 sites in the city to pull off [source: O’Neill].

Even in areas with very low humidity, there’s at least some water in the sky and clouds. A rainstorm happens after moisture collects around naturally occurring particles in the air, causing the air to reach a level of saturation at which point it can no longer hold in that moisture. Cloud seeding essentially helps that process along, providing additional “nuclei” around which water condenses. These nuclei can be salts, calcium chloride, dry ice or silver iodide, which the Chinese use. Silver iodide is effective because its form is similar to ice crystals. Calcium chloride is often used in warm or tropical areas.

Is cloud seeding just a way of sweeping pollution and climate change under the rug and playing God to ensure you don’t have to schedule a rain date? Or is it merely a matter of tweaking weather to your advantage? However you look at it, it’s not likely to go away any time soon. In fact, scientists have already proposed building fleets of massive, unmanned ships to seed clouds over Earth’s oceans to provide a cooling counter to carbon dioxide-induced global warming [source: Cartlidge]. If cloud seeding is ultimately a way to “fake it” when guests come to visit or crops need a sprinkle, do we really want a world where we have to manually adjust the planet’s atmosphere just to stave off environmental disaster?